1. Field of the Invention
The present invention relates to an infrared radiation panel.
2. Description of Related Art
Infrared radiation panels are known in the art and have been supplied by Kanthal AB, Sweden, among others.
Such panels are, in principle, constructed by mounting an electric resistor wire on a wall of ceramic fiber material. The resistor wire is connected to a source of current, so that the wire can be heated to high temperatures, for instance temperatures on the order of 1500-1600.degree. C. The resistor wire then emits infrared radiation.
One problem with these known panels is that the effective life of the resistor wire is not sufficiently long in relation to the desired effective life span of the panel. For instance, in the paper industry, where infrared radiation panels could be used to dry paper and paper pulp, a long effective life span is required because of the continuity of the manufacturing processes involved. For instance, the paper industry desires an effective life span of 16000 hours. Known panels that include a known resistor element and that are marketed by Kanthal AB under the name Kanthal Super 1800 have an effective life span of 6000 hours.
Electric resistor elements of the molybdenum silicide type have long been known. These resistor elements find use primarily in so-called high temperature applications, such as ovens that operate at temperatures of up to about 1700.degree. C.
Swedish Patent Specification 458 646 describes the resistor element Kanthal Super 1900. The material used is an homogenous material with the chemical formula Mo.sub.x W.sub.1-x Si.sub.2. In the chemical formula, the molybdenum and tungsten are isomorphous and can thus replace one another in the same structure. The material does not consist of a mixture of the materials MoSi.sub.2 and WSi.sub.2.
SiO.sub.2 grows on the surface of the heating element at a parabolic growth rate upon exposure to oxygen at high temperatures, this growth rate being the same irrespective of the cross-sectional dimensions of the heating element. The thickness of the layer may be 0.1 to 0.2 mm after some hundred hours in operation at a temperature of 1850.degree. C. When cooling down to room temperature, this glaze layer will solidify and subject the basic material of the heating element to tension forces owing to the fact that the coefficients of thermal expansion of the basic material differs significantly from that of the glaze. The coefficient of thermal expansion of the glaze is 0.5.times.10.sup.-6, whereas the thermal coefficient of expansion of the basic material is 7-8.times.10.sup.-6.
These tension forces will, of course, increase with increasing thicknesses of the glaze layer. When the tension forces exceed the mechanical strength of the basic material fractures will occur therein, which takes place when the glaze has grown above a certain critical thickness.
In the case of more slender elements, the proportion of the cross-sectional area constituted by the glaze in relation to the basic material will be larger than in the case of thicker elements. The critical glaze thickness will therefore be reached after a much shorter working time in the case of slender elements than in the case of thicker elements at the same working temperature and under the same operating conditions in general.
It has been believed hitherto that this has been the dominant factor in the effective life span of an infrared radiation panel.
It has been found, however, that the panel construction with respect to the attachment of the resistance wire is highly significant.
The present invention provides an infrared radiation panel whose effective life span is much longer than that of known panels when using the same resistance wire.